Dietary Comparisons of Adult Male Common Grackles, Red-winged Blackbirds, and Yellowheaded Blackbirds in North Central North Dakota

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1 University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln USDA National Wildlife Research Center - Staff Publications U.S. Department of Agriculture: Animal and Plant Health Inspection Service 1994 Dietary Comparisons of Adult Male Common Grackles, Red-winged Blackbirds, and Yellowheaded Blackbirds in North Central North Dakota H. Jeffrey Homan North Dakota State University George M. Linz Denver Wildlife Research Center, george.m.linz@aphis.usda.gov William J. Bleier North Dakota State University Robert B. Carlson North Dakota State University Follow this and additional works at: Part of the Environmental Sciences Commons Homan, H. Jeffrey; Linz, George M.; Bleier, William J.; and Carlson, Robert B., "Dietary Comparisons of Adult Male Common Grackles, Red-winged Blackbirds, and Yellow-headed Blackbirds in North Central North Dakota" (1994). USDA National Wildlife Research Center - Staff Publications This Article is brought to you for free and open access by the U.S. Department of Agriculture: Animal and Plant Health Inspection Service at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in USDA National Wildlife Research Center - Staff Publications by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln.

2 Prairie Naturalist 26(4): December Dietary Comparisons of Adult Male Common Grackles, Red-winged Blackbirds, and Yellow-headed Blackbirds in North Central North Dakota H. JEFFREY HOMAN, GEORGE M. LINZ, WILLIAM J. BLEIER, and ROBERT B. CARLSON Department of Zoology, North Dakota State University, Fargo, ND (WH, WJB) U. S. Department of Agriculture, Denver Wildlife Research Center, North Dakota Field Station, Fargo, NO (GML) Department of Entomology, North Dakota State University, Fargo, ND (RBC) ABSTRACT - We compared the esophageal contents of adult male common grackles (Quiscalus quiscula), red-winged blackbirds (Agelaius phoeniceus),and yellow-headed blackbirds (Xanthocephalus xanthocephalus) collected in north central North Dakota from JUly through October Temporal variation in the diets of all species was related to food availabilities, which were heavily influenced by crop phenology. Depending on species, mean percent dry weight of sunflower achenes during August increased 2-6 times over July. Yellow-headed blackbirds migrated from north central North Dakota by early September, but substantial numbers of common grackles and red-winged blackbirds remained until mid-october. Sunflower was an important component of red-winged blackbird and common grackle diets through October. The more varied diet of yellow-headed blackbirds, in combination with the early departure from sunflower-growing regions, makes this species less likely to cause economic impact to the sunflower producer. Key words: Agelaius phoeniceus, food habits, North Dakota, Quiscalus quiscula, sunflower, Xanthocephalus xanthocephalus Red-winged blackbirds (Agelaius phoeniceus), yellow-headed blackbirds (Xanthocephalus xanthocephalus), and common grackles (Quiscalus quiscula) are the most abundant blackbird species (lcterinae) in North Dakota. Comparisons between breeding bird surveys made in 1967 and 1991 indicated that red-winged blackbirds decreased 68% to 1.4 million breeding pairs, whereas yellow-headed blackbirds increased 225% to 665,000 pairs and common grackles increased 105% to 698,000 pairs (Stewart and Kantrud 1972, Nelms 1991). Blackbirds aggregate into large mixed-species flocks beginning in July and forage in habitats surrounding their wetland roosting sites until migration in September and October (Linz 1982). Economic losses to North Dakota sunflower

3 274 Prairie Naturalist 26(4): December 1994 producers from foraging blackbirds were estimated at $2 million annually from 1986 to 1991 (Huffman 1992). The birds apparently prefer sunflower achenes (sunflower) to other available foods because of the high energy content of sunflower and its availability near wetland roosting sites (Otis and Kilburn 1988). Research on the food habits of male and female red-winged blackbirds, yellow-headed blackbirds, and common grackles has shown that male red-winged and yellow-headed blackbirds and both sexes of common grackles were primarily responsible for sunflower damage in North Dakota (Linz et al. 1983, 1984; Twedt et al. 1991; Homan et al. 1994). However, these studies were conducted in different areas of North Dakota and during different years. Thus, direct dietary comparisons among species were not possible. We investigated the feeding habits of adult male red-winged blackbirds, yellow-headed blackbirds, and common grackles in Benson County, North Dakota, from 1 July to 25 October Our objectives were to compare the diets among the three species in crop and noncrop habitats and to analyze temporal dietary variation. STUDY AREA Benson County (area 364,900 ha) is located in north central North Dakota in the northeastern Drift Plain physiographic region (S tewart 1975). The terrain is flat to gently rolling and is primarily cropland interspersed with numerous shelterbelts and potholes. Approximately 206,275 ha (56%) of Benson County were cultivated in 1989 (North Dakota Agricultural Statistics Service 1990); 66% of the cropped land was planted to small grains (wheat and barley). Sunflower, oats, and com accounted for 10%,4%, and 2% of the hectares, respectively. Minor crops (e.g., rye, flaxseed, edible beans, and soybeans) represented less than 3% of the hectares planted in Hay was grown on 12% of the cultivated land. The average annual precipitation is 43 cm, with the majority falling from April through September. In summer, the average temperature is 19 C. Average dates of first and last frosts (0 C) are 13 September and 23 May, respectively. METHODS Collecting and Processing We collected adults of the three blackbird species by shotgun one to two times weekly in Benson County, North Dakota, from 1 July to 25 October Toreduce the influence of time of collection and habitat, the birds were collected during all daylight hours wherever and whenever encountered. Additionally, we collected birds from flightlines in the late afternoon while they were going to roost. The birds were numbered, placed in plastic bags, and immediately packed in ice to retard postmortem esophageal digestion of foods. We either processed the birds within four hours of collection or froze them for later processing.

4 Homan et al.: Blackbird Feeding Ecology 275 Each bird was weighed and aged using a combination of plumage characteristics and the presence or absence of the bursa of Fabricius (Wright and Wright 1944). The food items were removed from the oral cavity and esophagus, including the proventriculus (Gartshore et al. 1979), placed in ethanol, and sorted as sunflower, com, small grains (wheat, barley, oats), weed seeds [largely foxtail (Setaria spp.), wild oats (Avena spp.)], insects, grit, and unidentifiable matter (Homan et al. 1994). All foods were oven-dried at 70 0 C for hr, air cooled, and weighed to the nearest g on an electronic top-loading balance. Statistical Analyses Only birds containing one or mqre food item(s) were included in the analyses (Homan et al. 1994). Food items weighing less than g were recorded as g to indicate that food was present. After deriving the mean proportion esophageal dry weights ofthe individualfood categories (Swanson et al. 1974), we omitted the com and unidentifiable matter categories because of extremely low frequencies of occurrence. Grit was not included in the analyses. We used multivariate analysis of variance (MANOY A) on square-root, arcsine-transformed mean proportions of esophageal dry weight (Zar 1984: ) to compare diets among habitats (sunflower and noncrop), species, and months. Low availabilities of yellow-headed blackbirds in Benson County in September and October precluded dietary analyses among the three species for these months; thus, only the diets of red-winged blackbirds and common grackles were compared during this time. In sunflower habitat, esophageal contents of the three species were analyzed only for August; we could not collect a large enough sample size to analyze all three species' diets in July. F-statistics, based on Wilk' s lambda, refer to simultaneous comparisons of all dietary categories (SAS Institute, Inc. 1988). Statistical significance was accepted atps;0.05. Preliminary dietary analyses between birds from sunflower and non crop habitats (shelterbelts, pastures, wetlands, and flightlines) revealed a habitat effect (p<o.oool); thus, habitats were analyzed separately. Significant interactions (P'sS;0.05) occurred between species and months; therefore, diets were compared within months. Food consumption is reported as mean percent dry weight and relative percent frequency of occurrence (Twedt et al. 1991, Homan et al. 1994). The mean percent dry weight for each food category is defmed as the average of the dry weight proportions multiplied by 100 (Swanson et al. 1974). The relative percent frequency of occurrence is the number of times a food category occurs divided by the total number of times all food categories occur in all individuals multiplied by 100 (Twedt et al. 1991, Homan et al. 1994).

5 276 Prairie Naturalist 26(4): December 1994 RESULTS Noncrop Habitats Esophageal contents differed among the three species for July (F=2.54; df=8,102; P=O.Ol) and August (F=3.29; df=8,226; P=O.OOl). In July, red-winged blackbirds consumed more sunflower and weed seeds but fewer insects than yellow-headed blackbirds (Table 1). Also in July, more insects were found in common grackles than in red-winged blackbirds. The esophageal contents of yellow-headed blackbirds and common grackles were similar during July, with insects comprising approximately 50% of both species' diets. In August, redwinged blackbirds contained fewer small grains and weed seeds and more sunflower than yellow-headed blackbirds, whereas common grackles consumed more sunflower and fewer weed seeds than yellow-headed blackbirds (Table 1). Redwinged blackbirds and common grackles had similar esophageal contents in August, w hen both species dramatically increased their use of sunflower. However, esophageal contents were different in September (F=4.62; df=4,71; P=0.OO2) and October (F=20.68; df=4,67; P<O.OOOl) between the two species. In September, more weed seeds were found in red-winged blackbirds than in common grackles (Table 1). In October, the differences in the diets of the two species were more pronounced, with red-winged blackbirds consuming fewer insects and small grains but more sunflower; weed seeds were not found in common grackles in October. Sunflower Habitat Common grackles and red-winged blackbirds consumed more sunflower and fewer weed seeds in August than yellow-headed blackbirds (F=8.54; df=8,412; P<O.OOOl). Common grackles and red-winged blackbirds had similar diets in August and concentrated on sunflower almost exclusively (Table 2). There was a significant difference in dietary intake in September, when common grackles consumed fewer weed seeds than red-winged blackbirds (F=3.19; df=4,226; P=O.Ol), but sunflower consumption by both species still remained high. In October, red-winged blackbirds displayed a more varied diet than common grackles, who relied almost completely on sunflower (Table 2); during October, redwinged blackbirds ate less sunflower but more insects and weed seeds than common grackles (F= 11.04; df=4,54; P<O.OOO 1). DISCUSSION We compared the esophageal contents of adult male red-winged blackbirds, yellow-headed blackbirds, and common grackles collected in the same locale during the same time period. Variation in the males' diets was similar to that found in studies conducted elsewhere in North Dakota that focused on sex differences in red-winged blackbirds (Linz etal. 1983, 1984), yellow-headed blackbirds (Twedt et al. 1991), and common grackles (Homan et al. 1994). Temporal variation in the

6 Table 1. Monthly comparisons of mean percent dry weight'! (relative percent frequency of occurrence b ) of esophageal food ; contents of male red-winged blackbirds (Ap), common grackles (Qq), and yellow-headed blackbirds (Xx) collected in noncrop habitats C in Benson County, North Dakota, in ~ Foods Month July August September Ap Qq Xx Ap Qq Xx Ap Qq n = 19 n = 10 n =28 n = 69 n = 16 n =34 n =46 n =30 Sunflower 33 (26) 10 (10) 4 (4) 67 (47) 52 (48) 24 (16) 55 (30) 42 (39) Small grains d 10 (6) 20 (20) 29 (22) 17 (15) 35 (32) 46 (26) 35 (22) 47 (36) Weed seeds e 41 (36) 20(20) 18 (18) 11 (18) 6 (10) 26 (29) 8 (20) trf (2) Insects 16 (32) 50(50) 49 (56) 5 (20) 7 (10) 4 (29) 2 (28) 11 (23) "Me.. percent cry weight. weight of bod + weight of au bod categories determined lor each bird, ti*i a_aged over all birds and muliplied by 100. i>relative percent frequency of ocruitence number of birds containing a bod item + the total number of ocrurrences of au bod items in au birds and mukiplied by 100. cshelterbehs, wetlands, pastures and woodlands. "wheat, barley and oats. "Primarily foxtail (Setari. spp.) and wild oats (Awn. spp.). "'.<O.S%. October Ap Qq n =55 n = (51) 44 (33) 1 (1) 52 (42) 10(40) o (0) tr (8) 4 (25) I o 3 ~ ::J n ~ a or a ~. ~ o 0- ~..., 'I 'I

7 278 Prairie Naturalist 26(4): December 1994 Table 2. Monthly comparisons of mean percent dry weigh~ (relative percent frequency of occurrence b ) of esophageal food contents of male red-winged blackbirds (Ap), common grackles (Qq), and yellow-headed blackbirds (Xx) collected in sunflower fields in Benson County, North Dakota, in Foods Month August September October Ap Qq Xx Ap Qq Ap Qq n = 130 n = 19 n = 63 n=118 n=113 n = 44 n =30 Sunflower 92 (68) 98 (86) 62 (43) 93 (53) 94 (74) 67 (36) 100(94) Small grains C 2 (2) o (0) 1 (1) 2 (3) 3 (5) 1 (4) o (0) Weed seeds d 4 (10) o (0) 36 (33) 3 (16) 1 (5) 30 (32) o (0) Insects 2 (20) 2 (14) 1 (23) 2 (28) 2 (16) 2 (28) tr e (6) "Mean percent dry weight = weight of food + weight of all food categories determined for each bird, then averaged over all birds and multiplied by 100. brelalive percent frequency of occultence _ the number of birds containing a food item + the total number of OCCUlTenC8S of.11 food items in all birds and mukipliedby 100. <Wheat, barley and oats. dprimarily foxtail (Setaria spp.) and wild oats (Avena spp.). "tr = <0.50/0. diets was related to food availabilities, which in turn were heavily influenced by crop phenology (Twedt et al. 1991, Homan et al. 1994). In addition to the influences exerted by crop maturation, the habitats in which the birds were collected contributed significantly to dietary variation. Because of this habitat effect, we conducted dietary analyses separately between sunflower and noncrop habitats. Noncrop Habitats Blackbird damage to sunflower is greatest in August (Cummings et al. 1989), when sunflower is in the "milk stage" of development. In August, yellow-headed blackbirds used small grains, weed seeds, and sunflower in decreasing order of importance. They consumed sunflower in proportions less than half that of common grackles or red-winged blackbirds collected during the same period. Yellowheaded blackbirds usually migrate from north central North Dakota by early September (Twedt et al. 1991), before common grackles and red-winged blackbirds. The latter two species will often feed in sunflower until migration in late September and October. The migratory timing and diet of yellow-headed blackbirds make them less likely to cause a significant economic impact to sunflower producers in North Dakota. Considering the availabilities of crops in the county, the birds were utilizing sunflower (10% of the cropped land) over small grains (67% of the cropped land). Small grains were an important food for blackbirds from July through October.

8 Homan et al.: Blackbird Feeding Ecology 279 However, most small grains consumed after July were probably waste grains from harvested fields. Relative percent frequencies of occurrence for insects and weed seeds demonstrated that they were important dietary items for all three blackbird species (Table 1). The departure between the August values for relative percent frequency of occurrence and mean percent dry weight reflects the increasing availabilities of sunflower and small grains due to crop maturation. Both of these foods have heavier per unit weights than either weed seeds or insects. The mean percent dry weight value probably underestimates the dietary importance of weed seeds and insects. Cattail-dominated wetlands are often used by red-winged blackbirds as roosting and loafing sites, with small wetlands adjacent to sunflower appearing to be particularly attractive. Sunflower losses can be extensive near these wetlands (Otis and Kilburn 1988). Red-winged blackbirds were often encountered and collected while loafing in small (less than 1 ha) cattail-dominated wetlands adjacent to sunflower. Common grackles were usually collected in more upland-type habitats, such as shelterbelts, pastures, and harvested grain fields. This difference in diurnal habitat use between the two species probably accounts for some of the dietary variation in October, when red-winged blackbirds collected in noncrop habitats contained 89% sunflower compared to 44% for common grackles. There appears to be only small between-year dietary variation in consumption of sunflower for adult male red-winged blackbirds in North Dakota. During August, estimates of sunflower consumption by red-winged blackbirds collected in noncrop habitats was 67% in 1980 (Linz et al. 1983) and 69% (gross dry weight) for our study. In contrast, comparisons with our data revealed that adult male yellowheaded blackbirds collected from noncrop habitats in Benson County during August 1987 consumed approximately twice as much sunflower (Twedt et al. 1991). Benson County in 1987 had 14% of its cropped land planted in sunflower, 40% more than in The increased availability of sunflower in 1987 or the effects on food availabilities from the late-1980s drought could account for the variability between years in yellow-headed blackbird diets. Sunflower Habitat During August, yellow-headed blackbirds had a more diverse diet than redwinged blackbirds and common grackles, which both fed almost exclusively on sunflower during August and September. Only in October did sunflower consumption differ between the two species; during October, red-winged blackbirds increased their weed seed consumption to 30%, while common grackles avoided weed seeds. As the availability of standing sunflower began to decrease, alternate foods were probably being sought, thus causing a change in the red-winged blackbird's diet. Furthermore, the hardening of the fibrous hulls of the matured sunflower achenes may have decreased the shelling efficiency of the smaller-billed red-winged blackbirds, causing a shift in food use (Mah and Nuechterlein 1991). The lack of weed seeds in the diet of common grackles may be related to the

9 280 Prairie Naturalist 26(4): December 1994 energetic inefficiency of such relatively large birds (> 100 g) pursuing small-sized and often widely -spaced foods. In the presence of corn, Dolbeer et al. (1978) noted that common grackles rarely fed on weed seeds. The high frequencies of occurrence of insects found in red-winged blackbirds and common grackles in September were probably the result of an infestation of sunflower seed weevils (Smicronyx spp.) in Benson County (Twedt et al. 1991). Incidental ingestion of the larval weevils was more likely than a purposeful preying upon these insects, as the larvae bore into the achenes and are probably not visible from the outside (Linz et al. 1984). Because of its abundant Class IV wetlands and sunflower fields, North Dakota will continue to have large breeding and transient populations of blackbirds. The yellow-headed blackbird's more varied diet and early migration from the sunflower-growing region probably make this species the least likely of the three most abundant blackbird species to cause a significant economic impact to the sunflower producer. ACKNOWLEDGMENTS We thank D. Bergman, D. Foulk, K. Maruskie, B. Osborne, S. Redlinger, K. Scheidecker, and M. Soehren for their help in the field and lab. D. Twedt assisted with the study design and statistical analyses. We thank the landowners of Benson County for access to their properties. Our research was supported by the Denver Wildlife Research Center, Denver, CO (Contract No OO20(CA)) and the Department of Zoology, North Dakota State University, Fargo. LITERATURE CITED Cummings, J. L., J. L. Guarino, andc. E. Knittle Chronology of blackbird damage to sunflowers. Wildl. Soc. Bull. 17: Dolbeer,R. A.,P.P. Woronecki,A.R.Stickley,andS.B. White Agricultural impact of a winter population of blackbirds and starlings. Wilson Bull. 90: Gartshore, R. G., R. J. Brooks, J. D. Somers, and F. F. Gilbert Temporal change in gullet passage in penned red-winged blackbirds (Agelaius phoeniceus): significance for research in feeding ecology. Can. J. Zool. 57: Homan, H. J., G. L. Linz, and W. J. Bleier Effect of crop phenology and habitat on the diet of common grackles. Am. Midl. Nat. 131: Huffman, L. E Magnitude and potential solutions of blackbird-sunflower problem. Pp. 47 in Proceedings cattail management symposium (G. M. Linz, ed.). U. S. Department of Agriculture/Animal and Plant Health Inspection Service/Animal Damage Contro1/Denver Wildlife Research Center and U. S. Department of Interior/Fish and Wildlife Service, Fargo, ND.

10 Homan et al.: Blackbird Feeding Ecology 281 Linz, G. M Feeding and molt of the red-winged blackbird. Ph.D. Thesis. North Dakota State University, Fargo. Linz, G. M., D. L. Vakoch, J. F. Cassel, and R. B. Carlson Food of redwinged blackbirds collected at a roost in late summer in Cass County, North Dakota. Prairie Nat. 15: Linz, G. M., D. L. Vakoch, J. F. Cassel, and R. B. Carlson Food of redwinged blackbirds (Age/aius phoeniceus) in sunflower fields and com fields. Can. Field-Nat. 98: Mah, J., and G. L. Nuechterlein Feeding behavior of red-winged blackbirds on bird-resistant sunflowers. Wildl. Soc. Bull. 19: Nelms, C. O Population estimates and comparison of sampling methods of breeding blackbirds in North Dakota. M.S. Thesis. North Dakota State University, Fargo. North Dakota Agricultural Statistics Service Agriculture Statistics No. 58. North Dakota State University, Fargo. Otis, D. L., and C. M. Kilburn Influence of environmental factors on blackbird damage to sunflower. U. S. Fish Wildl. Tech. Rep. 16. SAS Institute, Inc SAS/STAT user's guide: Release 6.03 ed. SAS Institute, Inc., Cary, NC. Stewart, R. E Breeding birds of North Dakota. Tri-college Center for Environmental Studies, Fargo, ND. Stewart, R. E., and H. A. Kantrud Population estimates of breeding birds in North Dakota. Auk 89: Swanson, G. A., G.L. Krapu, J. C. Bartonek, J. R. Serie, and D. H. Johnson Advantages in mathematically weighting waterfowl food habits data. J. Wildl. Manage. 38: Twedt,D.J., W.J. Bleier, and G. M.Linz Geographic and temporal variation in the diet of yellow-headed blackbirds. Condor 93: Wright, P. L., and M. H. Wright The reproductive cycle of the male redwinged blackbird. Condor 46: Zar, J. H Biostatistical analysis, 2nd ed. Prentice-Hall, Inc., Englewood Cliffs, NJ. Received 27 June Accepted 12 September 1994.